Heat-labile anti-complementary activity (ACA) appears in normal human serum during storage or heating as endogenous haemolytic activity disappears. Following gel filtration of unheated serum, two peaks of heat-labile ACA are present. The ACA of both whole and fractionated serum has previously been attributed to the presence of heat-labile immunoglobulin aggregates or immune complexes. Our data demonstrate that the heavy peak of ACA obtained by gel filtration does not bind to 125I-C1q or to Raji cells, and that its effect is abolished to C1INH, suggesting that it represents C1 rather than immunoglobulin aggregates or immune complexes. The lighter peak of ACA in fractionated serum has the functional characteristics of C1s and free C1s is demonstrable in fractions containing this activity. The ACA of whole serum likewise has functional characteristics of C1. The anti-complementary effect of C1 on guinea-pig complement would not be evident in the complement fixation assay until most endogenous haemolytic activity in human serum has been inactivated, either by heat or by storage. C1INH only partially inhibits this ACA in serum or in solutions containing isolated C1 in high concentrations. These observations indicate that heat-labile ACA in whole or fractionated sera is due to the presence of C1 and C1s and that this activity cannot be taken as evidence for the presence of immune complexes.